Terahertz emissions (THz) are generally defined in the frequency band located between 0.3 THz and 10 THz. Their generation and their detection are of great interest given the many applications that they enable such as for example the detection of pollutants and dangerous materials, surveillance imaging, medical imaging, THz communications, etc.
The emission of THz emissions may be obtained directly using emission sources, for example backward wave oscillators (BWOs), which are also known as “Carcinotrons™”, molecular lasers or certain solid-state lasers such as quantum-cascade lasers.
It is also possible to obtain THz emissions indirectly using nonlinear conversion in nonlinear crystals of emissions of more accessible frequencies such as radiofrequencies, microwave frequencies, visible or infrared frequencies. The processes implemented are for example frequency multiplication, optical rectification or even heterodyne mixing.
The article “Frequency Stabilized GaP Continuous-Wave Terahertz Signal Generator for High-Resolution Spectroscopy” by Sasaki et al (Optics and Photonics Journal, 2014, 4, 8-13) thus presents an indirect THz source based on use of two tunable laser sources in the near infrared. The beams output from the two laser sources are spatially superposed using a plate beamsplitter and illuminate a nonlinear crystal of gallium phosphide (GaP) in order to generate a THz emission the frequency of which corresponds to the frequency difference between the frequencies of the two incident laser beams. The resultant THz frequency may be tuned by changing the frequency of one of the two tunable laser sources. This technique allows a tunable THz emission to be produced at room temperature.
The present description also presents an indirect THz laser source that, as in the aforementioned article, is based on difference-frequency generation and that, with respect to known devices, has an excellent frequency stability and a THz emission of extremely precise frequency.